A significant factor limiting the length of optical fiber transmission links is attenuation. In silica-based optical fiber design and manufacture, the attenuation performance of optical fibers has improved from about 20 dB/km in 1970 to near the theoretical minimum today: about 0.35 dB/km at 1310 nm and about 0.2 dB/km at 1550 nm.
In addition, optical fiber amplifiers using rare-earth dopants are well known, and recently various commercial systems have become available. See, e.g., Armitage, "Three-level fiber laser amplifier: a theoretical model", APPLIED OPTICS, Vol. 27, No. 23, Dec. 1, 1988, and the references cited therein. These fiber amplifiers can substantially reduce the link-length limitations due to fiber attenuation. To date, the only practical fiber amplifiers operate around 1520-1565 nm, where there is a transition in the Er.sup.3+ dopant ion.
Another important link-length limitation is the total chromatic dispersion which occurs due to the material dispersion and waveguide dispersion in the optical fiber which forms the transmission link. Dispersion causes pulse spreading for pulses that include a range of wavelengths, as the speed of light in a glass fiber waveguide is a function of the wavelength of the light. Pulse broadening is a function of fiber dispersion, fiber length and the spectral width of the light source. Even systems with very narrow source wavelength range are affected by chromatic dispersion because the wavelength range of all sources is spread to some extent, for example, due to laser source chirp.
In standard step index single-mode fibers, the graph of total chromatic dispersion versus wavelength is largely a function of material dispersion and can be drawn as a roughly linear curve with positive slope. The curve has a zero crossing at approximately 1310 nm, and reaches a value of approximately 15 ps/nm-km at 1550 nm. Agrawal, Nonlinear Fiber Optics, Academic Press, Inc., San Diego, Calif., 1989, p. 11. For such conventional fibers, the bandwidth is highest around 1310 nm where dispersion is approximately zero. These conventional fibers are said to be optimized for operation around 1310 nm (hereinafter "optimized for operation at 1310 nm).
On the other hand, the minimum theoretical attenuation for conventional single-mode fibers made of GeO.sub.2 --SiO.sub.2 glass is in the region of 1550 nm, and is due to Rayleigh scattering and infrared absorption. When transmission is carried out at 1310 nm, standard step-index fibers have attenuation approximately 1.75 times the theoretical minimum at 1550 nm. For transmission at 1550 nm, where standard step-index fibers generate substantial dispersion, link length is dispersion limited, as dispersion effects outweigh attenuation benefits.
As the total performance of the fiber is a function of both dispersion and attenuation, various attempts have been made to minimize the dispersion at 1550 nm in order to take advantage of the minimum attenuation in this wavelength range. Numerous "dispersion shifted" fiber designs have been developed which shift the zero crossing of the dispersion vs. wavelength curve to the 1550 nm region. See, e.g.: Cohen, Lin and French, ELECTRONICS LETTERS, Vol. 15, No. 12, Jun. 7, 1979, pp. 334-335; Bhagavatula U.S. Pat. No. 4,715,679; Saifi et al., "Triangular-profile single-mode fiber", OPTICS LETTERS, Vol. 7, No. 1, Jan. 1982, pp. 43-45; Ohashi et al. U.S. Pat. No. 4,755,022; Bhagavatula, "Dispersion-shifted and dispersion-flattened single-mode designs", Technical Digest, Conference on Optical Fiber Communication, paper WF1, Feb. 26, 1986; and, Tanaka et al., "Low-Loss Dispersion Shifted Fiber with Dual Shape Refractive Index Profile", National Conference Record 1987, Semiconductor Devices and Materials, I.E.I.C.E. (1987), p. 2-217. These dispersion shifted fibers are based on special refractive index profiles which generate negative waveguide dispersion to shift the total dispersion vs. wavelength curve to the right (to longer wavelengths).
In addition, numerous "dispersion flattened" fibers have been designed which have zero dispersion crossings in both the 1300 nm and 1550 nm transmission regions. See, e.g.: the Bhagavatula patent and paper cited above; Okamoto et al., "Dispersion Minimization in single-mode fibres over a wide spectral range", ELECTRONICS LETTERS, Vol. 15, No. 22, Oct. 25, 1979, pp. 729-731; Okamoto et al. U.S. Pat. No. 4,525,027; Cohen et al. U.K. patent 2 116 744; Cohen et al., "Low-loss Quadruple-clad single-mode lightguides with dispersion below 2 ps/km-nm over the 1.28 .mu.m-1.65.mu.m wavelength range", ELECTRONICS LETTERS, Vol. 18, No. 24, Nov. 25, 1982, pp. 1023-1024; Cohen et al., "Ultrabroadband single-mode fibers", Technical Digest, Conference on Optical Fiber Communication, paper MF4, Feb. 28, 1983; Cohen et al., "A systematic approach to fabricating single-mode lightguides", Proc SPIE, Vol. 425 (1983), pp. 28-32; Sears et al., "Measurements of the axial uniformity of dispersion spectra in single-mode fibers", Proc SPIE, Vol. 425 (1983), pp. 56-62; Unger U.S. Pat. No. 4,691,991; Francois, "Propagation Mechanisms in Quadruple-clad fibres: mode coupling, dispersion and pure bend losses", ELECTRONICS LETTERS, Vol. 19, No. 21, Oct. 13, 1983, pp. 885-886; and, Shigematsu et al. EPO published patent application 0 283 748.
Some references indicate that dispersion flattened fibers also have the benefit of a reduced slope around the zero crossing, enabling low dispersion transmission over a relatively wide range of wavelengths near the transmission wavelength. See, e.g.: Okamoto et al. U.S. Pat. No. 4,372,647; and, Lazay et al. U.S. Pat. 4,439,007.
Some dispersion flattened fiber designs generate slightly negative total dispersion at wavelengths in the range from 1300 nm to 1550 nm. Bhagavatula et al., "Segmented-core Single-mode Fibres with Low Loss and Low Dispersion", ELECTRONICS LETTERS, Vol. 19, No 9, Apr. 25, 1983, pp. 317-318, depicts in FIG. 3, a dispersion flattened fiber design, C, which has a total dispersion versus wavelength curve varying from about -5 ps/km-nm at 1300 nm to about -2 ps/km-nm at 1550 nm. This dispersion flattened design would not be practicable for dispersion compensation at 1550 nm, as the length of dispersion compensating fiber required would be 7-8 times the length of the transmission fiber. A similar dispersion versus wavelength curve is mentioned in Reed U.S. Pat. No. 4,852,968, at col. 9, lines 25 -30.
Cohen et al., "Tailoring the shapes of dispersion spectra to control bandwidths in single-mode fibers", OPTICS LETTERS, Vol. 7, No. 4, Apr. 1982, pp. 183-185, is directed to dispersion flattening in computer-simulated "double-clad" fibers. FIG. 6, on page 185, includes one simulated fiber design, Case 3, which the authors claim "indicates that the short-wavelength zero crossing can conceivably be moved to a wavelength shorter than the material-dispersion zero crossing" (par. 1). In addition to shifting the zero crossing to the left, this simulated design indicates a very steeply negative slope in the 1450 nm wavelength region, and a total chromatic dispersion in this region of less than -40 ps/km-nm. Cohen et al. U.S. Pat. No. 4,435,040 contains a parallel disclosure, at col. 6, lines 45-54, with respect to FIG. 6.
It is not clear what, if anything, this reference indicates with respect to the 1550 nm region. If the dispersion curve is simply extended with a ruler, the value at 1550 nm would go off the measurement scale of FIG. 6 by a factor of several times the entire scale. There is no disclosure or suggestion with respect to the 1550 nm region in the reference, as the Case 3 simulation is directed to creating a zero crossing at a wavelength less than 1310.
What is clear from the Cohen et al. "Tailoring . . ." reference is that the absolute value of the slope of the total dispersion curve (about 2 ps/nm.sup.2 -km) is much greater than the slope of the total dispersion curve for standard singlemode fiber optimized for transmission at 1310 nm (which is about 0.06 ps/nm.sup.2 -km). It is thought that a fiber of this design will not transmit light at 1550 nm. Such a fiber would have very large attenuation, much greater than 1dB/km. This simulated fiber would not be practicable as a dispersion compensating fiber in the 1550 nm window (approximately 1520 nm-1565 nm) for a number of reasons. First, this fiber would not transmit light in the 1550 nm region, as the bend-edge wavelength for an actual fiber which might display such a highly negatively sloped dispersion vs. wavelength curve would be significantly below 1520 nm. The bend-edge wavelength is the wavelength at which a straight fiber will no longer propagate the fundamental mode.
Second, even a slight variation in transmission wavelength would result in an enormous change in dispersion compensating effect and therefore in the length of dispersion compensating fiber required for canceling out the positive dispersion in a conventional 1310 nm transmission link. In addition, the authors acknowledge the difficulty of manufacturing these "conceivable" simulated fibers: "As one might expect, the potentially attractive properties of double-clad light guides require tight tolerances on diameter and index difference" (p. 185, col. 1, par. 1).
Techniques have been proposed for transmission links with dispersion compensation means to cancel out the total chromatic dispersion over the link. Kogelnick et al. U.S. Pat. No. 4,261,639 is directed to an optical pulse equalization technique for minimizing pulse dispersion in a single-mode fiber transmission system. (See also, Lin, Kogelnick and Cohen, "Optical-pulse equalization of low-dispersion transmission in single-mode fibers in the 1.3-1.7 .mu.m spectral region", OPTICS LETTERS, Vol. 5, No. 11, November 1980, pp. 476-478.) In the Kogelnick system, the negative dispersion of a transmission fiber is canceled out by the positive dispersion of an equalizer fiber. The lengths of the two fibers are matched based on the ratio of their dispersion values.
The example given in Kogelnick (col. 4, lines 26-56) is transmission of a signal with 5 nm spectral width at 1550 nm over a 100 km transmission fiber with minimum dispersion at 1560 nm, resulting in a dispersion of (-0.8 ps/nm-km) (100 km) (5nm)=-400 psec. A 1350 nm single mode fiber is intended as the equalizer fiber, and since its dispersion at 1550 nm would be approximately 16 ps/nm-km, a 5 km length of equalizer fiber provides a dispersion of 400 psec, and thereby cancels out the total dispersion over the combined link to zero.
In the Lin, Kogelnick and Cohen paper (p. 477, cited above), the example given is a transmission link with a 1 km fiber having zero dispersion at 1510 nm, and a 0.76 km fiber with zero dispersion at 1320. The zero crossing of the total dispersion curve for the combined fibers is measured at 1420 nm.
The system of Kogelnick presents serious problems. For small differences between a transmission fiber's zero dispersion wavelength and the source wavelength, a relatively short length of commercially available equalizer fiber may be used, as explained in the Kogelnick patent. However, as presented in the Lin, Kogelnick and Cohen paper, for large wavelength differences, increasingly long lengths of equalizer fiber are required, and the link-length becomes attenuation limited. Thus, the Kogelnick idea is unworkable in solving the primary problem to which the present invention is directed: a practicable transmission system utilizing a 1550 nm source over a transmission fiber with zero dispersion at 1310 nm.
A similar system, with the same drawbacks is described in Larner and Bhagavatula, "Dispersion Reduction in Single-mode-fibre links", ELECTRONICS LETTERS, Vol. 21, No. 24, Nov. 21, 1985, pp. 1171-72. In this system, 1 km and 2.5 km of standard single mode fiber with zero dispersion at 1310 nm are added to a 60 km link of dispersion shifted fiber with zero dispersion at 1550 nm, to shift the wavelength of zero dispersion for the link toward the source wavelength, 1541 nm. Improved transmission performance was measured and graphed.
Tick et al. U.S. Pat. No. 4,969,710 is directed to yet another dispersion compensation technique, the use of a fluoride glass based fiber to compensate for dispersion in a SiO.sub.2 -based optical fiber. The zero dispersion wavelength is approximately 2000 nm for the fluoride glass based fiber. In the hypothetical example given, a 1 km SiO.sub.2 based standard transmission fiber with zero dispersion at 1320 nm is combined with a 0.454 km fluoride glass fiber with zero dispersion at 2000 nm to achieve zero dispersion for the combined link at the transmission wavelength, 1550 nm (see col. 6, lines 24-36 and col. 7, lines 48-59). Using the standard dispersion convention, the dispersion of the standard fiber at 1550 nm would be about 15 ps/km-nm, and therefore that of the fluoride glass fiber would be about -33 ps/km-nm. (Note, due to a difference in sign convention for the definition of dispersion, the graph of dispersion versus wavelength is upside down in FIGS. 4 and 5 of the Tick et al. patent; for the purposes of the present application, all references shall be to the sign convention and dispersion equation conventionally used in the U.S., as set forth in Agrawal, Nonlinear Fiber Optics, Academic Press, Inc., San Diego, Calif., 1989, p. 10.)
Although this technique allows for a shorter length of fluoride glass fiber than the equalizer fiber of Kogelnick et al., it is disadvantageous in that fluoride glass fibers as required in Tick et al. are not commonly available at present.
Byron EPO published patent application 0 089 655 is directed to fibers made of fluoride glass (62 HfF.sub.4 -33 BaF.sub.2 -5 LaF.sub.3) which have zero material dispersion crossing at about 1600 nm (see FIG. 3). FIG. 3 of Byron suggests that dispersion values as low as -10 ps/km-nm may be possible with dispersion shifted fiber profiles using such fluoride glass.
Numerous other dispersion compensation techniques have been considered in the prior art. Bhagavatula U.S. Pat. No. 4,750,802 is directed to a fiber delay line array for dispersion compensation. Bhagavatula U.S. Pat. No. 4,768,853 is directed to a dispersion compensation system using a segment of multimode fiber as a dispersion transformer. Kafka U.S. Pat. No. 4,913,520 is directed to a pulse compression technique using self-phase modulation to compress a laser output pulse width. Agrawal et al. U.S. Pat. No. 4,979,234 is directed to a pulse compression technique using a saturated semiconductor laser amplifier.
As discussed above, a primary problem to which the present invention is directed is the design and implementation of a transmission system utilizing a source at a given wavelength within the 1550 nm wavelength window over a transmission fiber with zero dispersion at about 1310 nm. This issue is very important commercially, as the overwhelming majority of fiber transmission links installed today are based on optical fiber with zero dispersion at about 1310 nm. The upgrading of existing standard single mode 1310 optimized optical fiber routes to higher capacities is an issue of great concern to long-haul telecommunication providers.
For example, for a 1550 nm system transmitting over a standard step-index single mode fiber, the fiber's positive dispersion level of about 15 ps/km-nm causes distortion of 40 channel (or greater) 50-500 MHz AM video signals and limits link-lengths to less than 5 km. Vodhanel et al., "Performance of Directly Modulated DFB Lasers in 10-Gb/s ASK, FSK, and DPSK Lightwave Systems", JOURNAL OF LIGHTWAVE TECHNOLOGY, Vol. 8, No. 9, September 1990, pp. 1379-1385 indicates that in a 10-Gb/s transmission experiment using direct intensity modulation of a 1550 nm DFB laser and direct detection, transmission over a standard 1300 nm optimized fiber is limited to a length of 3 km before unacceptable bit error rate occurs.
Ideally, the upgrade method for existing 1310 nm optimized links would include both increased data rates and the bypassing of electronic regenerator sites (via longer span capability), so that the existing fiber could be used more efficiently with a minimum of new equipment. In the 1550 nm wavelength window (approximately 1520 nm-1565 nm), erbium-doped optical fiber amplifiers (OFAs) can effectively remove the attenuation loss limitation. However, in the 1310 nm wavelength window, optical fiber amplifiers are not available.
Gnauck et al., "Optical Equalization of Fiber Chromatic Dispersion in a 5-Gb/s Transmission System", IEEE PHOTONICS TECHNOLOGY LETTERS, Vol. 2, No. 8, August 1990, pp. 585-587, is directed to a dispersion equalization technique using a reflective Fabry-Perot interferometer. The system used 64.5 km of standard single-mode fiber with dispersion of 17 ps/km-nm at 1530 nm. The reference acknowledges a loss of 6 dB in the equalization process, but states that the loss could be compensated by optical amplification or reduced by using an optical circulator (col. 2, lines 1-5).
Gysel, "CATV AM Optical Transmission Links Using the 1550 nm Window", Proc. Manual, Fiber Optics 1991, Society of Cable Television Engineers, January 1991, pp. 161-166, is directed to an electrical dispersion compensation circuit which compensates for dispersion generated by 1550 nm transmission over single mode fiber with zero dispersion at 1310 nm. This electronic technique is limited by the narrow operating wavelength range of the filter devices it relies on.
In view of the above-noted problems with dispersion compensating techniques, there remains an important commercial need for a fiber based dispersion compensating system. It is an object of the present invention to provide an all fiber optical transmission link with minimal limitation on link-length due to fiber attenuation and total chromatic dispersion. Another object of the present invention is a simple fiber-based, all-optical dispersion compensation technique that essentially permits 1310 nm optimized fiber to operate in the 1550 nm wavelength window as if it were dispersion-shifted fiber, resulting in a substantial increase in transmission bandwidth and/or reduction in Composite Second Order (CSO) distortion.
A further object of the present invention is an erbium doped optical fiber amplifier system operating at 1550 nm with highly negative dispersion, so that the system can be combined with a transmission link having positive dispersion at 1550 nm, in order to provide dispersion compensation without added attenuation.
It is a further object of this invention to provide a dispersion compensating SiO.sub.2 --GeO.sub.2 glass fiber which provides highly negative total chromatic dispersion in the wavelength range from 1520 nm to 1565 nm. It is a further object of the present invention to provide such a fiber which can be used in relatively short lengths to compensate for the dispersion in standard length links of fiber optimized for transmission at 1310 nm. Yet another object of the present invention is such a dispersion compensating fiber with low attenuation, not greater than 5 times the attenuation per km of commercially available standard fiber, and preferably less than 3 times.
A further object of this invention is the creation of fibers with total dispersion versus wavelength curves whose slopes are controlled to provide dispersion flattening for a particular link in the 1550 nm wavelength range. This feature is described herein as "slope compensation", and it allows the creation of a relatively broad wavelength window for either multiplexing several signals on one fiber or for greater latitude on the average wavelength and spectral width of the transmitting laser. In one embodiment of the present invention, the slope of the dispersion curve is in the range from 0 to -1.2 ps/nm.sup.2 -km.